<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
<style>
    .b1{
        width: 100px;
        height: 100px;
        background-color: yellow;
        border:10px solid red;
        /*样式： solid实线 double双实线 dashed虚横线 dotted虚圆点线*
            border-top/bottom/left/right
            可以拆分成border-style/width/color
        */
    }
</style>
</head>

<body>
    <div class="b1">

    </div>
</body>
</html>